dos33fsprogs/games/sb/zx02_optim.s
2023-03-10 00:28:36 -05:00

164 lines
3.9 KiB
ArmAsm

; De-compressor for ZX02 files
; ----------------------------
;
; Decompress ZX02 data (6502 optimized format), optimized for speed and size
; 138 bytes code, 58.0 cycles/byte in test file.
;
; Compress with:
; zx02 input.bin output.zx0
;
; (c) 2022 DMSC
; Code under MIT license, see LICENSE file.
;ZP=$80
;offset = ZP+0
;ZX0_src = ZP+2
;ZX0_dst = ZP+4
;bitr = ZP+6
;pntr = ZP+7
; Initial values for offset, source, destination and bitr
;zx0_ini_block:
; .byte $00, $00 ; offset
;comp_data:
; .byte $0, $0 ; zx0_src
;out_addr:
; .byte $0, $0 ; zx0_dst
; .byte $80 ; bitr
;--------------------------------------------------
; Decompress ZX0 data (6502 optimized format)
; destination page in A
full_decomp:
sta ZX0_dst+1
ldy #$80
sty bitr
ldy #0 ; always on page boundary
sty ZX0_dst
sty offset
sty offset+1
; Y needs to be 0 here
; Get initialization block
; ldy #7
;copy_init: lda zx0_ini_block-1, y
; sta offset-1, y
; dey
; bne copy_init
; Decode literal: Ccopy next N bytes from compressed file
; Elias(length) byte[1] byte[2] ... byte[N]
decode_literal:
jsr get_elias
cop0: lda (ZX0_src), y
inc ZX0_src
bne plus1
inc ZX0_src+1
plus1: sta (ZX0_dst),y
inc ZX0_dst
bne plus2
inc ZX0_dst+1
plus2: dex
bne cop0
asl bitr
bcs dzx0s_new_offset
; Copy from last offset (repeat N bytes from last offset)
; Elias(length)
jsr get_elias
dzx0s_copy:
lda ZX0_dst
sbc offset ; C=0 from get_elias
sta pntr
lda ZX0_dst+1
sbc offset+1
sta pntr+1
cop1:
lda (pntr), y
inc pntr
bne plus3
inc pntr+1
plus3: sta (ZX0_dst),y
inc ZX0_dst
bne plus4
inc ZX0_dst+1
plus4: dex
bne cop1
asl bitr
bcc decode_literal
; Copy from new offset (repeat N bytes from new offset)
; Elias(MSB(offset)) LSB(offset) Elias(length-1)
dzx0s_new_offset:
; Read elias code for high part of offset
jsr get_elias
beq exit ; Read a 0, signals the end
; Decrease and divide by 2
dex
txa
lsr ; @
sta offset+1
; Get low part of offset, a literal 7 bits
lda (ZX0_src), y
inc ZX0_src
bne plus5
inc ZX0_src+1
plus5:
; Divide by 2
ror ; @
sta offset
; And get the copy length.
; Start elias reading with the bit already in carry:
ldx #1
jsr elias_skip1
inx
bcc dzx0s_copy
; Read an elias-gamma interlaced code.
; ------------------------------------
get_elias:
; Initialize return value to #1
ldx #1
bne elias_start
elias_get: ; Read next data bit to result
asl bitr
rol ; @
tax
elias_start:
; Get one bit
asl bitr
bne elias_skip1
; Read new bit from stream
lda (ZX0_src), y
inc ZX0_src
bne plus6
inc ZX0_src+1
plus6: ;sec ; not needed, C=1 guaranteed from last bit
rol ;@
sta bitr
elias_skip1:
txa
bcs elias_get
; Got ending bit, stop reading
exit:
rts